1. Field of the Invention
The present invention relates to an ultrasonic testing method for detecting an internal flaw of articles.
2. Related Art Statement
The ultrasonic test has been generally used for detecting an internal flaw in steel members, steel sheets or plates, welded portions, machine parts and other articles. Recently, the ultrasonic test also has been used for detecting an internal flaw in ceramics and like new materials. However, the conventional ultrasonic testing method decides a condition of a detected flaw of a test article by presuming a size of the flaw from a size of a flaw echo reflected from the flaw measured by means of an oscilloscope or the like, or by presuming a size of the flaw from a size of a two dimensional indication of the flaw detected by automatically scanning the test article. However, even if the internal flaws in the test articles are the same in size, influences on strength of the articles are entirely different dependent on the condition of the flaws. Particularly, in a brittle material such as ceramics, the influences of flaws are remarkably different dependent on the condition of flaws and therefore there was a problem with presuming the condition of the flaw by only the size of the indication. For example, when the flaw is decided on only the size, an article having a flaw which is in large size but does not actually affect the strength of the article is rejected, on the contrary, an article having a crack of small size, but having a problem on the strength is accepted.
Furthermore, in the conventional ultrasonic testing method, an immersing prove having a simple plane transducer has been used for transmitting an ultrasonic wave to a test article immersed in water such as a steel member, steel sheet, forged article or the like, since the test article is a relatively large and the flaw also is larger than several millimeters. Moreover, there has been used a focusing probe having a transducer provided with a concave resin lens or a transducer formed into a concave shape in order to detect a flaw of several hundred .mu.m, when the article requires a high reliability. Recently, there have been developed high reliable articles made of brittle structural materials such as ceramics to be used under a severe circumstance. Accordingly, it is desired to establish a high analyzable testing method for deciding the internal flaw in the articles. In the ultrasonic test for detecting the internal flaw in ceramic article or the like, use is made of an ultrasonic wave of a high frequency such as 15.about.100 MHz higher than the conventional testing ultrasonic wave of 0.5.about.10 MHz, and a computer is also frequently used for processing wave pattern.
However, the prior art technique mentioned above can be applied only to articles having simple configurations such as a flat sheet or plate, cylindrical body, and cylindrical surface, but articles having a curved surface, particularly curved surface of a radius of curvature not larger than 10 mm is less detectable. For example, when the conventional probe having the flat transducer or the focusing probe is applied for the curved surface having a small radius of curvature, the ultrasonic wave is scattered at the incident curved surface so that the flaw is not detected. There are recently developed ceramic articles such as balls for bearing and ceramic parts for engine and gas turbine, and therefore a testing method for effectively detecting these articles having a curved surface of a small radius of curvature is desired.